1. Field of the Invention
This invention relates to a driving force transmitting apparatus, and in particular to a driving force transmitting apparatus which is used in a sheet winding apparatus of a copying machine.
2. Description of Related Art
Generally, driving force transmitting apparatus, such as clutches, etc. are used in many apparatus. Such driving force transmitting apparatus transmit rotational torque adjusted to a driven mechanism. For example, a driving force transmitting apparatus as described above is used in a color copying machine in which a microcapsule carrying sheet is used in U.S. Pat. No. 4,806,982, U.S. Pat. No. 4,827,356 and U.S. Pat. No. 4,847,661. In such a copying machine, a continuous microcapsule carrying sheet wound in a cartridge is provided and then wound onto a winding shaft (or take-up shaft) after its use. While being unwound, a latent image corresponding to an original manuscript is formed on the microcapsule carrying sheet by exposing a part of the microcapsule carrying sheet to a light image corresponding to the image of the manuscript. Further, the exposed part of the microcapsule sheet is pressed with a developer sheet carried from a developer sheet cassette in a pressure developing unit and a color image corresponding to the latent image is formed on the developer sheet. In this manner, color copying is completed.
In the above copying process, if the microcapsule sheet is overreleased, the latent image of the manuscript is often distorted as formed on the microcapsule sheet. This leads to copying of distorted incorrect images on the developer sheet. Therefore, in the above conventional copying machine, it is necessary to give a high tension to the microcapsule carrying sheet by rotating the winding shaft with high torque in order to obtain the correct latent image of the manuscript.
As shown in detail in FIG. 6, a first plate 43a is fixed to an end of a winding shaft 41 around which the microcapsule sheet is wound (not shown) and a second plate 43b for receiving driving force is connected to the first plate 43a through a compressing spring 42 located therebetween. A friction pad 44 is fixed to the second plate 43b. A third plate 46 for transmitting driving force is fixed to an end of a driving shaft 45 which is driven by a motor. Another friction pad 47 is fixed to the third plate 46 so as to contact with the friction pad 44. By this construction, driving force from the driving shaft 45 is transmitted to the winding shaft 41 through frictional power caused between the second and the third plates. As a result, the microcapsule sheet is wound by rotation of the winding shaft 41.
In the above described conventional driving force transmitting apparatus, in order to transmit driving force with high torque it is necessary: (1) to enlarge sizes of both the second and third plates, resulting in an increase of the frictional area between both the pads 44, 47; (2) to strengthen the compressing force of the compressing spring 42 disposed between the first and second plates; or (3) to form the pads 44, 47 from materials which have a large frictional coefficient. However, the above methods (1), (2) and (3) have the following defects: the first method (1) requires a large space due to the enlarged sizes of the second and third plates and thus prevents the manufacture of a compact copying machine; the second method (2) is difficult to assemble and an undesired influence is produced on the mechanical strength of the apparatus due to the strengthened compressing force of the compressing springs 42; and the third method (3) is not economical because the life of the pads 44 and 47 is decreased due to the material having the large frictional coefficient causing sand burning of the pads 44, 47.